Buoyant device in crystal growing



Feb. 3, 1970 G. DESSAUER 3,493,343

BUOYANT DEVICE IN CRYSTAL GROWING Filed July I, 1966 CRYSTAL PULLlNGDRIVE INVENTOR RALPH G. DESSAUER AT TORNE Y United States Patent3,493,348 BUOYANT DEVICE IN CRYSTAL GROWlNG Ralph G. Dessauer, Beacon,N.Y., assignor to International Business Machines Corporation, Armonk,N.Y., a corporation of New York Filed July 1, 1966, Ser. No. 562,214Int. Cl. Btllj 17/20 US. Cl. 23273 8 Claims ABSTRACT OF THE DISCLOSUREThis invention relates to a buoyant force device which is effective inmoving a shaft by means of its buoyant characteristic and moreparticularly to the use of such device in a system for automaticallydrawing an elongated crystal from a melt of material.

Crystals have been grown from melts for many years. However, therequirement for large quantities of high quality crystals has not comeabout until quite recently. This requirement is particularly needed inthe semiconductor industry. Drawing large single crystals to closediameter tolerances is an important phase of the mass production ofsemiconductor devices.

One of the most widely used techniques in drawing semiconductor singlecrystals is the Czochralski pulling method. The high puritysemiconductor material is melted in a container and the temperature ismaintained just above the freezing point of the material. A particularlyoriented seed crystal is then dipped into the melt and the seed isslowly raised from the melt. The melt liquid adheres to the seed bysurface tension and adhesive forces, and under the correct conditionsthe crystal will grow as it is slowly pulled away from the melt.

The diameter of the crystals grown by the Czochr-alski method is afunction of many varying conditions. Close and constant attention byhighly skilled operators is required to obtain crystal diameters withinacceptable limits. A solution to this operator problem in the form of acontrol system for automatically drawing an elongated crystal from amelt of material is described in patent application Ser. No. 530,819(IBM Docket 14,472) entitled Control System, filed Mar. 1, 1966 by R. G.Dessauer, E. J. Patzner and M. R. Poponiak and assigned to the sameassignee as the present invention. In this control system a radiationdetector is utilized for sensing radiation propagating from the melt andfor providing an output proportional to the amount of radiation sensed.The output of the radiation detector is applied to a means for adjustingthe growth condition of the crystal. Adjustment of the growth conditionis accomplished by means of adjusting the crystal pulling mechanism, thecontainer lift, the container rotation rate or combinations of thesemechanisms. While this invention is a substantial improvement over theprior art, it is not readily possible to maintain the melt at a constantlevel with respect to the heater means. The importance of maintainingthe melt at a constant heater position is for uniform heating. Further,Where used with the above referred to patent application, the radiationdetector Will 'ice always be focused onto the melt-crystal interfacebecause the melt level will not change.

It is thus an object of the present invention to provide a new buoyantforce device for moving a shaft in response to a heavier or lighterforce at the other end of the shaft.

It is another object of the present invention to provide an improvedsystem for growing elongated crystals from a melt of material.

It is a further object of this invention to provide a system for growingelongated crystals wherein a buoyant force device is used to maintainthe melt-crystal interface at a constant level during the entire crystalgrowing period.

These objects are accomplished in accordance with the broad aspects ofthe present invention by providing at one end of a shaft a means whichis responsive to a change in weight of the article at the other end ofthe shaft to maintain the level of the article at the other end of theshaft. This is accomplished by use of a buoyant force mechanism whichincludes a float suspended from one end of the shaft in a flotationfluid. The flotation fluid. The flotation fluid is located within atank. The weight of the float and other assembly parts is supported byuse of constant force spring means which may be attached to the tank.

The use of the buoyant force device in an apparatus for growing acrystal from a melt is particularly valuable. A container holding themelt is secured to the upper end of a vertically movable shaft and atthe lower end is the means for vertically moving the shaft as thecrystal grows to maintain the melt-crystal interface at a constantlevel. Surrounding the container is a heater which maintains themelt-crystal interface just above the melting point of the material inthe container. It is important that this interface level be maintainedat a uniform level level. The melt-crystal interface level will alwaysbe at the same location since the container is gradually raised as themelt is used up during the crystal growth due to the buoyant force ofthe device. The melt-crystal interface is therefore held at a uniformlevel with respect to the heater means which allows the temperature ofthe meltcrystal interface to be maintained almost precise the sameduring the entire crystal growth period. This greatly increases thequality of the crystal grown in that it reduces imperfections therein.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of the preferred embodiments of the invention as illustratedin the accompanying drawing.

In the drawing:

FIGURE 1 is a partially in cross-section, schematic view of theapparatus of the present invention.

Referring now to FIGURE 1 there is shown an apparatus for growingsemiconductor crystals such as silicon single crystals using the buoyantforce device of the present invention. A silicon charge 10 is held in acontainer or a crucible 12 composed of a suitable material such as fusedquartz. A suitable heater means such as resistance heater 14 is providedto maintain the silicon charge in its melted form, and just above thefreezing point of the silicon at its melt-crystal interface. Thecontainer 12 is supported on the end of a vertically movable shaft 16.The shaft alternately may also be movable rotationally and vertically asillustrated in the drawing. Means (not shown) are provided for holding acrystal seed on the end of crystal pulling shaft 18. The seed is dippedinto the melt and withdrawn therefrom to grow a crystal 20. The crystalpulling means 18 serves both as the support for the crystal and forraising the crystal as it is being grown. A crystal pulling drive means22 is used to slowly raise the crystal pulling means 18. The furnace oroven bell jar 24 encloses the container and the heating means.

The means for vertically moving the shaft 16 as the crystal grows is thebuoyant force device 30. The buoyant force device 30 includes a floatmeans 32 attached to the shaft 16 for purposes of vertically moving theshaft 16. The float 32 is located in a flotation fluid 34 which issupported in a tank 36. The float means 32 is flexibly attached to thecover plate 38 of the tank means 36 by means of constant force means 40which are used to support the approximate weight of the float means 32,the shaft 16, and the container 12. The illustrated means 40 are flatcoil constant force springs. There may be two or more of these constantforce spring means 40 to support the weight or float means, shaft andcontainer in the flotation fluid 34.

As the weight of the melt is reduced by the crystal growth of crystal20, the float means .2 gradually is raised and it raises the shaft 16which in turn raises the container 12 and in turn maintains themeltcrystal interface at a uniform level in relation to the heater means14.

It is preferred in the Czochralski crystal growing method to have thecontainer rotate during crystal growth. Means 50 are therefore providedfor rotating the shaft 16 and, in turn, the container 12. Such arotating means may include a driven gear 52 which drives external shaft54. The shaft 16 is connected to the external shaft 54 by a ball spline56 which allows the vertical motion of the shaft 16 and at the same timeallows the external shaft 54 to rotate the shaft 16. The external shaft54 is illustrated as attached by screw means 58 to the tank coverplate38. The tank and the float means 32 are therefore also rotated in thisembodiment. It will be obvious to one skilled in the art that the tankand float means need not be rotated and appropriate modifications in thelinkages could be made to accomplish this result.

The buoyant force mechanism is preferably positioned within a coolingjacket 60 so as to maintain the density of the flotation fluid at thedesired value. This is important since the flotation fluid densityaffects the buoyant force of the float means 32. This is particularlyimportant where cooling fluids such as perfiuoroamine and ethers areused which have a steep temperature density curve. Thesefluoro-chernicals have higher densities than water and therefore can beused in conjunction with a smaller float means and other assembly parts.Provision can be made for an appropriate inlet and outlet (not shown)for circulating cooling water through the cooling Water jacket 60.

A suitable sensing unit, shown schematically as 65, can sense how farthe float means has risen and interpret this into the quantity of meltmaterial remaining in the container 12 and the melt level with respectto the container. This information is continuously available to anoperator on an indicator device (not shown). With this information, themaximum crystal growth can be obtained. Alternately, a linear velocitytransducer can be used to give an indication of the rise velocity of thefloat. This is a continuous indication of crystal diameter where aconstant crystal pull rate is maintained. Therefore, the output of thetransducer can be used, for example, to adjust heater temperature to inturn control crystal diameter.

Centrifugal effects on the fluid level are small with the low rotationalvelocity usually used. They may, if neces sary, be counteracted byadjusting the tank elevation when the float is rotating at operationalvelocity.

The relationship required to achieve the objective of continuouslyraising the container at a rate equal to that of the melt level drop inthe container may be derived for constant cross-sectional tank floatmeans and container. As the float rises out of the fluid, the fluidlevel drops and partially negates the rise of the float. The rise of thefloat 4 can, of course, never be totally negated since in that case thesystem will have returned to the initial conditions.

The design equation for the present device is determined by thefollowing. It is required that there be no vertical displacement of themelt level relative to the tank, Ad Therefore:

Ad t o AW mp...

Considering the tank and float only, by Archimedes" principle the changein melt weight, AW, can be described in terms of the verticaldisplacement of the float in the fluid, Ad the cross-sectional area ofthe float, A and flotation fluid density, p as:

flo fPo Using this equation and considering that the total volume offlotation fluid remains constant during the operation, the verticaldisplacement of the float in the tank can be ex- ;pressed in terms ofthe change in melt weight, flotation fluid density, and the tank andfloat cross-sectional areas, A, and A respectively, as:

The Equations 1 and 2 may be combined and the final design equationresults:

1 1 1 &

For different size containers 12 or different density melts 10 theweight change per inch of rise will differ. This can be accommodated byadjustment according to the design equations. This adjustment is readilyaccomplished changing the area of the tank A, by means of the additionor removal of rods 70. The apparatus is thus a spring with a variablespring constant.

From the foregoing description, it will be seen that the buoyant forcedevice is relatively simple in construction and is useful where auniformly variable vertical force of variable rate is required withoutfatigue and friction inherent in springs in general. The device isuseful where it is desired to keep the surface level elevation of afluid or a powder constant in a container as fluid is added to orsubtracted from the container. The device has been shown as a greatvalue, particularly as a component part of an automated crystal growthsystem.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

1. apparatus for growing a crystal from a melt comprising:

a container for holding said melt;

a heater means having a uniform heat level at the melt-crystal interfacesurrounding said container; crystal pulling means for raising the saidcrystal as it is being grown;

a vertically and rotationally movable shaft supporting said container;

means for constantly rotating said shaft and, in turn,

said container; means for vertically moving said shaft as the crystalgrows to maintain said melt-crystal interface at said uniform heatlevel; and

said means for vertically moving includes a float means attached to saidshaft which float means is suspended in a floatation fluid within a tankso that the said shaft and said container are slowly moved upward bybuoyant force as the crystal grows.

2. The apparatus of claim 1 further comprising constant force meansattached to said tank for substantial 1y supporting the weight of saidfloat means, said shaft and said container so that only the weight ofsaid melt is supported by said buoyant force.

3. The apparatus of claim 1 further comprising means for changing thecross-sectional area of said tank.

4. An apparatus for growing a crystal from a melt comprising:

a container for holding said melt;

crystal pulling means for raising the said crystal as it is being grown;

a vertically movable shaft supporting said container;

a float means attached to said shaft; and

said float means being suspended in a flotation fluid within a tank sothat the said shaft and said container are slowly moved upward bybuoyant force as the crystal growth continues and the melt-crystalinterface is maintained at a constant level.

5. The apparatus of claim 4 wherein said shaft is additionallyrotationally movable and further comprising means for rotationallymoving said shaft and, in turn, said container.

6. The apparatus of claim 4 further comprising means for changing thecross-sectional area of said tank.

7. An apparatus for growing a crystal from a melt comprising:

a container for holding said melt;

a heater means having a uniform heat level at the melt-crystal interfacesurrounding said container;

2. seedholder for drawing a seed from the container;

crystal pulling means attached to said seed-holder for raising the saidcrystal as it is being grown;

means for advancing and rotating the container as the melt thereindepletes;

said means for advancing and depleting includes a splined shaftsupporting the container mounted with an external shaft for rotationtherewith about the shaft axis and for axial movement therein;

means for rotating the external shaft;

tank means with a liquid having a level therein fixed to said externalshaft and rotatable therewith;

said spline shaft extending into said tank and having a float mountedthereon;

the said float secured to said tank through a constant force spring;

and said float responding to a depletion of the melt for advancing thespline shaft axially upward through the external shaft.

8. The apparatus of claim 7, further comprising means for changing thecross-sectional area of said tank.

References Cited UNITED STATES PATENTS 2,944,875 7/1960 Leverton 232733,033,660 5/1962 Okkerse 23273 3,268,297 8/1966 Fischer 23273 3,340,0169/1967 Wirth 23273 2,872,299 2/1959 Celmer 23301 2,934,409 4/1960 Biehl23-267.1 2,976,129 3/1961 Buehler 23267.1 2,979,386 4/1961 Shockley23301 3,246,504 4/1966 Halff et al. 7345l 3,359,077 12/1967 Argt 2330'12,728,123 12/1955 Jordan 222-259 FOREIGN PATENTS 1,080,071 4/ 1960Germany.

906,152 5/ 1945 France.

NORMAN YUDKOFF, Primary Examiner US. Cl. X.R, 73319; 137-403

